Treating inherently precipitationhardenable chromium-nickel stainless steel



- atented Feb. 26, 1946 TREATING INHER-ENTLY PRECIPITATION- HARDENABLECHROMIUM-N ICKEL STAINr' LESS STEEL Robert H. Aborn, Short mus; N.J.,,assignor to United States Steel Corporation of Delaware, 7

a corporation of Delaware a Na Drawing. Application December 10, 1943,

Serial No. 513,748

.2 Claims. (Cl. 14s-21.55)

This invention is a method of treating chromium-nickel stainless steelcontaining carbon and at least one of the'elements, such as titanium,columbium, vanadium, etc., which form a carbide more stable thanchromium carbide. This type of steel usually contains aluminum, and theamount of those alloying elements which are ferrite formers other thanthe named carbide formers is proportioned to the amount of thosewhichare austenite formers so as to 30% titanium and .15% aluminum. Thesteel was rendered completely austenitic except for some stable deltaferrite by heating it to 2050 F..

after which it was cooled in the furnace to 250 F. in forty-two hours,it thereafter being air cooled to room temperature, the steel havingbeen in 18 gage sheet form so that air cooling provided bring about theformation of stress-laden martensite at a relatively low temperaturesuch as room temperature; when the steel cools from a relatively hightemperature at which it is austenitic. This stress-laden martensiticstructure of the metal is inherently precipitationhardenable andincreases in hardness to a level depending on the temperature to whichit is reheated and the time at temperature; but prior to this reheating,the structure has suflicient ductility to permit some degree of coldforming of the metal.

The present invention is a method of treating steel of this type byconverting it to austenite and then cooling it in such a way that itremains temporarily as an austenitic or austenitic-like structure whenit reaches room temperature. This temporary structure is more ductileand permits a far greater degree of cold forming than theprecipitation-hardenable martensitic-like condition to which "itultimate transforms, but only after an interval long enough to permitthe completion of the desired forming operations.

According to this invention the steel is heated to austenitize it and isthen cooled slowly to temperatures slightly above those producing thestress-laden transformation product, whereupon it is cooled more rapidlyto room temperature at a rate preventing any substantial amount oftransformation, whereby the steel remains austenitic or in anaustenitic-like condition for a substantial time at room temperature.During perature and the conditions of cooling or if desired, thetransformation may be hurried by exposure to other temperatures.

As a specific example of the invention, the treatment has been appliedto the steel when it contains 17% chromium,7% nickel, .06% carbon,

an adequately fast 'cooling'method. By such treatment test reportsshowed that a maximum Olsen value of 0.500" wasreadily obtained,'thenormal Olsen value being 0.250" when the steel was austenitized at 1900F. and air cooled to room temperature.

In making the commercial form of this steel, it will have onlyapproximately the range of alloying elements noted, with the usualbalance of iron and impurities, and in such instances the austenitizingtemperature may range from 1700" to 2000 F., and the temperature towhich the ing, or, in the case of sections having such mass as to renderthis inadequate, by water quenching. After the treatment the steel issubstantially nonmagnetic, since it appears to be essentiallyaustenitic, and at ordinary room temperature it may remain unchanged fora period of several hours. If the steel is water quenched to roomtemperature from the high temperature where it is'fully austenitic,except for some delta'fer rite which may be present, the resulting structure may then be mainly austenitic but it will remain in this conditiononly a few minutes at most. It follows that the slow cooling to atemperature slightly above that at which the austenite would normallytransform to the marte nsitic or martensitic-like structure isnecessary, the transformation then being prevented by adequately fastcooling to room temperature. 5

. I claim: I i

1. A method of treating chromium-nickel stainless steel containingcarbon and at least one of the stronger-than-chromium carbide formerssuch as titanium, columbium, vanadium, etc.,

with its components that are ferrite formers other than said carbideformers proportioned to those that are austenite formers to cause theproduction of an inherently precipitation-hardenablestress-ladenmartensite or martensitic-like structure at low temperatures when thesteel cools to room temperature from high temperatures where it isessentially austenitic, said method comprising heating the steel toaustenitize it, cooling it slowly to temperatures slightly above thoseproducing said martensite or martensitic like structure, and thenrapidlycooling the steel' to'room' temperature at a rate preventing anysubstantial degree of transformation, whereby the steel remainsessentially austenitic for a substantial time at room temperature.

2. A method of treating chromium-nickel stainless steel approximatelycontaining 17% chromium, 7% nickel, .06% carbon, .8 0 titamum, and .15%aluminum, with its components that are ferrite formers proportioned tothose that are austenite formers to causethe production or stress-ladenmartensite or martensiticaccuses like product,

ROBERT H. ABORN.

